The present invention relates broadly to tobacco curing methods and associated apparatus and, more particularly, to an apparatus and method for facilitating a process for curing tobacco and providing an associated apparatus for controlling air circulation.
Globally, people smoke fifteen billion cigarettes everyday, with United States accounting for approximately one billion cigarettes consumed each day.
The two leading tobacco-producing states are Kentucky and North Carolina with North Carolina producing primarily flue-cured bright leaf tobacco while Kentucky produces primarily air-cured burley tobacco. The western mountains of North Carolina also produce burley tobacco.
Nearly half of all tobacco farms are located in Kentucky where the annual tobacco crop has averaged more than eight hundred million dollars during the decade of the 1990's. Tobacco currently accounts for around fifty percent of Kentucky's crop receipts and twenty five percent of Kentucky's total agricultural cash receipts, yet tobacco uses one percent of the farmland in Kentucky. An acre of tobacco averages around four thousand dollars in gross returns at the farm level while contributing around forty thousand dollars in federal, state and local tax revenue as a result of excise taxes on tobacco products. Average yields are about 2,500 pounds per acre and the plants are stalk cut. The leaves are stripped after curing. In addition to Kentucky, burley tobacco is produced in Tennessee, Ohio, Virginia, North Carolina, West Virginia and Missouri, with Kentucky and Western North Carolina accounting for the majority of burley produced. Accordingly, tobacco has a significant economic impact on these states.
Burley tobacco is a light air-cured tobacco and accounts for about eleven percent of world production. Cured burley leaf is characterized by low sugar content and a very low sugar-to-nitrogen ratio. The low sugar-to-nitrogen ration may be enhanced by using high nitrogen fertilizer, harvesting at an early stage of senescence, and through the air-curing process that allows oxidation of any sugars which may have occurred.
Another feature of burley tobacco is its significant capacity to absorb flavorings. Burley can typically absorb twenty five percent of its own weight in flavorings in contrast to about seven or eight percent for flue-cured tobacco.
The cured burley leaves vary in color from light tan to red and brown and the leaf should be without yellow patches or fringes. The burley leaves are slightly larger than flue-cured leaves and the burley plants are generally taller. A typical burley plant is topped at 20-30 leaves.
As is generally known, tobacco must be cured before it is used to manufacture cigarettes, or other tobacco products. Curing is the process that brings about the rapid destruction of chlorophyll, giving leaves their yellow appearance, converting sugar into starch and removing moisture. Curing brings out the aroma and flavor of each variety of tobacco. Before tobacco is cured, the leaves contain about 80 to 85 percent water which is essentially eliminated in the curing process. Among the factors that affect the curing process are growth soil, position of the leaf on the stalk and weather, including the environment within the curing barn.
Curing involves essentially three steps including yellowing, leaf drying and stem drying. The yellowing stage is a continuation of the ripening process and some say the most important part of the curing process. During yellowing, the leaf remains alive which allows it to carry on certain biological processes to convert sugar into starch and to break down chlorophyll. The stomata allows a continuous exchange of carbon dioxide, oxygen and water to the leaf. As the exchange evolves, yellow pigments become visible as chlorophyll breaks down.
When tobacco leaves are harvested, they are high in sugar and low in starch. As sugar decreases through the natural process of hydrolysis and respiration, the amount of starch increases. During the leaf drying stage, the leaf tissue is dried to predetermined moisture level. The stem drying stage is referred to as the killing stage because all moisture is removed from the stem and leaf, thereby killing them.
As stated above, burley tobacco is air-cured. Air-curing barns typically include an open framework including support members from which the whole stalk is hung and protected from wind and the sun. Barns may be equipped with ventilators that can be opened or closed to vary temperature and humidity to a limited extent.
A typical air-curing process takes six to eight weeks. Leaves turn from green to yellow as stems and leaves slowly dry. The leaves generally have a low sugar content and vary in nicotine content. Curing burley is not merely drying the leaf but is, in actuality, a six-to-eight week process that allows chemical changes to occur as the leaf dries. By varying temperature and humidity at the leaf, the quality of the product may be enhanced.
Good burley curing in a barn or field structure requires daily average humidity of about 65-75 percent to sustain enough leaf moisture for the necessary chemical changes to produce the desired tan and brown leaf colors. Cool fall temperatures below about 50 to 60 degrees are generally detrimental to good burley curing.
Consecutive days of higher or low humidity can also cause problems. For example, humidity above about 80 percent for three consecutive days in a warm environment can allow bacterial action to accelerate and cause rotting. Low humidity, i.e., below about 50 percent, for several days can desiccate the leaf moisture, stop chemical reactions and set undesirable green or yellow colors in the leaf. Therefore, control of airflow and humidity are important factors in controlling the curing process to provide commercial quality tobacco worthy of cigarette manufacture.
Dark air-cured tobacco accounts for about twenty percent of the world production and encompasses a number of types of tobacco used mainly for chewing, snuff, cigar and pipe blends. In the United States, dark air-cured tobacco is produced in Kentucky, Tennessee and Virginia. Three types are One-Sucker, Green River and Virginia sun-cured. This tobacco is characterized by heavy leaves. The plants are highly fertilized and topped low to around 10 to 12 leaves. Dark air-cured leaf is high in nicotine and is used in chewing and snuff and some pipe mixtures.
One-Sucker is the common name for a type of air-cured tobacco produced mainly in northern Tennessee, south central Kentucky and Indiana One-sucker dark air-cured tobacco is used in the manufacture of chewing tobacco. Green River is a type of dark air-cured tobacco produced principally in the Green River section of Kentucky around Owensboro and Henderson. It commonly used in chewing tobacco although to some extent it can be used for snuff and smoking tobaccos.
As stated above, bright leaf tobacco is flue-cured. Flue-cured tobacco is dried in a closed building with furnace-driven heat directed from flues or pipes that extend from a furnace into a barn. The temperature of the furnace is gradually raised until the leaves and stems are completely dried. For example, the temperature may be raised from 90 degrees Fahrenheit to 160 Fahrenheit. As the heat and humidity are controlled, moisture is removed, resulting in dried yellowed leaves and stems.
Flue-curing takes about a week and accounts for approximately fifty percent of the tobacco produced in the United States. Most flue-cured tobacco is used in the production of cigarettes and has a high sugar content and a medium-to-high nicotine content. Flue-cured tobacco is typically yellow to reddish-orange in color, thin to medium in body and mild in flavor.
Fire-cured tobacco is dried with low-burning wood fires on the floors of closed curing barns. The smoke is what gives fire-cured tobacco its smoky aroma and flavor. These leaves have a low sugar content but high nicotine content. Farmers regulate heat, humidity and ventilation in the curing barns. This process can be continuous or intermittent, extending from about three days to about ten weeks. Fire-cured tobacco is used to make cigarettes, chewing tobacco, snuff, and strong tasting cigars. Fire-cured tobacco is light-to-dark brown in color, medium-to-heavy in body and strong in flavor.
Of particular concern to tobacco producers and users are compounds known as tobacco-specific nitrosamines (TSNAs) which are carcinogenic. Some carcinogenic compounds that exist in tobacco smoke result from the combustion and pyrolysis of tobacco. Nitrosamines, on the other hand, are thought to be one of the only types of carcinogens that exist in both flue-cured and air-cured tobacco leaf.
Eight types of nitrosamines have been identified as tobacco-specific, seven of which have been reported as present in cigarette smoke. Since few nitrosamines are present in green tobacco leaf, the curing process plays a major role in the formation thereof. While it is not exactly clear how TSNAs form in air-cured burley, the current consensus is that bacteria or micro-flora convert nitrate to oxides of nitrogen that react with alkaloids to form TSNAs. Whether the reduction of TSNAs will have a significant effect on reducing the health risks of smokeless tobacco and cigarettes is not known. Nevertheless, reduction of nitrosamines through controlled curing environments can only have a positive effect on the tobacco product.
Due to the use of different curing methods throughout the world, the level of TSNAs in flue-cured tobacco varies. Most growers in the U.S., where flue-cured tobacco has some of the highest levels of TSNAs, use curing barns with direct-fired burners. In these barns, the exhaust gases of the burners are circulated through the barn, coming in contact with the tobacco. Combustion bi-products in the exhaust include oxides of nitrogen or NOx, which reacts with naturally occurring alkaloids in the tobacco to form TSNAs.
High TSNA tobacco is mainly a problem in the U.S., Canada, Australia and parts of Argentina where direct-fired curing is prevalent. Other major tobacco growing countries which typically have lower TSNA levels use heat exchanger-based curing, with either coal, wood or oil as their primary fuel. Heat exchanger curing excludes products of combustion from the curing air, reducing the likelihood of TSNA formation.
Direct-fired curing or fire-curing was not common in the U.S. until the late 1960s and early 1970s when the energy crisis occurred. Early heat exchanger equipped barns used wood or coal and eventually many farmers began using oil as it became increasingly expensive. Liquid propane gas was a cheaper and more readily available alternative so most growers converted their barns to use direct-fire burners fueled by propane.
There currently exists a need for unique and innovative tobacco curing processes that can be easily accomplished yet yield dramatic results, including the reduction of nitrosamines as well as the production of high quality tobacco.